Method of and apparatus for determining the oxygen content of metals



6ct.18,1966 P HOLLER 3,219,888

METHOD OF AND AI PARATUS FOR DETERMINING THE OXYGEN CONTENT OF METALS Filed Aug. 20, 1963 2 Sheets-Sheet l INVENTOR PAUL HOLLE R Oct. 18, 1966 P. HOLLER 3,279,888

METHOD OF AND APPARATUS FOR DETERMINING THE OXYGEN CONTENT OF METALS Filed Aug. 20, 1963 2 Sheets-Sheet 2 5 TE PP/NG DR/ VE LRrABSORP TION ANALYZER VALVE 9 STE PPING-DR/ V5 76 g TIMER RECORDER Fug. 2

' PAUL HOLLER INVENTOR AGENI United States Patent 3,279,888 METHOD OF AND APPARATUS FOR DETERMIN- ING THE OXYGEN CONTENT OF METALS Paul Holler, Essen-Frintrop, Germany, assignor to Hiittenwerk Oberhausen Aktiengesellschaft, Oberhausen, Rhineland, Germany, a corporation Filed Aug. 20, 1963, Ser. No. 304,719 Claims priority, application Germany, Aug. 21, 1962, H 46,701 2 Claims. (Cl. 23-230) My present invention relates to a method of determining the oxygen content of metals, especially ferrous metals such as steels and is a continuation-in-part of my copending application Ser. No. 193,229, filed May 8, 1962, now US. Patent No. 3,188,180, issued June 8, 1965.

The determination of the oxygen content of steels and other metals has been of great interest in the metallurgical field and several developments have recently permitted rapid analysis of steel samples during the refining process to give an indication of the impurity content of these samples at an early stage in the treatment of the metal. Even with these improved methods, it is difiicult to obtain a definite analysis in times shorter than five minutes so that there is almost invariably a serious lag, especially in the latter stages of refining, between testing of the melt and adjustment of the process.

In most of the earlier processes, the oxygen content of the metal is converted, by combination with carbon, to carbon monoxide which is then analyzed.

In my above-identified copending application, for example, I disclose and claim a method of extracting gases from metal samples with the aid of an electric arc wherein an extraction chamber is, prior to the introduction of a metal sample into the region of the arc, filled with an inert or reducing protective gas designed to eliminate any atmospheric oxygen which may be present in the chamber and to purge therefrom all traces of the residues from an earlier analysis. In the prior process, an electric arc is struck between the electrodes to produce a high temperature in excess of that to be employed subsequently for melting the sample, thereby releasing any impurity gases within the chamber and rendering the electrodes and/or the sample holders free from absorbed and adsorbed gases.

Advantageously, the atmosphere within the reaction chamber, in accordance with the principles of my application Ser. No. 193,229, consists of argon admixed with hydrogen in an amount between 1 and by volume. In this atmosphere, the purging of the extraction chamber can be carried out under reducing conditions with the aid of a higher energy electric arc than with the use of the pure inert gas. Subsequently, the metal samples are introduced into the chamber and blanketed with the pure inert gas, whereupon an electric arc is struck between the sample holder and a counterelectrode to free the gases trapped within the sample and, in the case of oxygen, produce carbon monoxide.

The extraction chamber is provided with a plurality of sample holders carried by a common support and successively displaceable into the region of the counterelectrode for degassing as described above. Generally, the analysis of the extracted gas is effected with the aid of a conventional spectrometer whose sensing element is directed at the electric arc. Preferably, the sample holders are mounted upon a turntable disposed within the extraction chamber.

While this technique is capable of relatively rapid analysis of metal samples (e.g., on the order of five minutes per sample), it has several disadvantages which are important in the metallurgical field. For one thing, the analysis time is not sufliciently low as to permit a high degree of monitoring of the melt in process. Additionally, the apparatus necessary for the spectrometric analysis of the melt is relatively expensive and requires constant observation by an attendant in most instances. Moreover, the spectrometric analysis method merely provides a relative measurement which must be compared with a calibration curve if absolute determinations are to be made. Moreover, these earlier systems are characterized by difiiculties to eliminate the getter" effect whereby a large proportion of the gases released from the samples are adsorbed or absorbed by the graphite electrodes, the soot-covered walls of the extraction chamber and by neighboring electrodes. The existence of cold portions of these electrodes and the chamber walls increases the getter efiect. This efiect is all the more inconvenient since it is not for the most part reproducible so that even when a calibration curve is provided for the spectrometric analysis apparatus, this curve is generally only valid for a particular set of circumstances and certain gettering conditions.

It is an object of the present inventionto provide a method of determining the oxygen content of metal samples in an extension of principles of my aforementioned copending application while obviating some of the disadvantages of earlier methods.

Another object of the present invention is to provide an apparatus for analyzing metal samples in an efficient and, preferably, automatic manner with a relatively short analysis time.

Still another object of the invention is to provide a method of automatically analyzing the oxygen content of steel samples wherein the disadvantages of gettering are eliminated.

These objects are attained, in accordance with the instant invention, by converting the substance, to which the analysis is directed and contained within a metal sample, into a gaseous .state by melting the sample. The gases within the extraction chamber are then admitted rapidly into a previously evacuated analysis chamber under the pressure difierenti-al existing between the two chambers. The gas is measured in the analysis chamber remote from the extraction chamber, so that any gettering in the latter, which is reduced by the rapid out-flow of the gas into a reduced pressure region, will have no eifect upon the actual analysis of the gas. According to a more specific feature of the invention, the melting of the. metal sample is carried out by disposing it within a hollow carbonaceous (i.e. graphitic) electrode which is juxtaposed with a counterelectrode and an electric discharge formed between the electrodes. Upon termination of the extraction step, without any maintenance of the discharge for purposes of spectrometric analysis, the extracted gas is transferred to the analysis or measuring chamber wherein it is studied by infrared absorption to determine the proportion of carbon monoxide present within the analysis chamber and, consequently, the oxygen content of a sample of a metal of known weight.

This method permits an absolute determination of the oxygen content in a steel sample in somewhat less than 2.5 min-utes because in several seconds a one-gram sample can be melted with the aid of an electric-arc discharge of between 20 and 40 amperes, the oxygen being quantitatively converted to carbon monoxide. Since the cloud of carbon monoxide surrounding the arc is drawn into the evacuated analysis chamber in a fraction of a second, there is no possibility that the walls of the vessel or the electrodes will absorb the gas by a getter effect, It is, consequently, an important aspect of this invention that the communication between the extraction chamber and the analysis chamber be established immediately upon termination of the arc discharge so that any tendency toward gettering is reduced if not entirely eliminated.

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According to another facet of this invention, the thorough purging of the extraction chamber by pumping the extracted gas therefrom or forcing it out of the latter in a carrier gas can be eliminated since there is a natural tendency for the gas to enter the previously evacuated analysis chamber and this takes place without the existence of a force stream of inert gas or passing the extracted gas through a pump. This result is important since the gas would otherwise adsorb readily upon the finely divided graphite within the evacuation chamber. Moreover, it should also be noted that it is important, in accordance with this invention, to terminate the electric arc immediately upon substantially complete extraction of the oxygen since further discharge results in convection currents between the discharge region and the walls of the vessel leading toward partial reabsorption of carbon monoxide by cooling portions of the metal vapor.

Since the extracted gas is immediately transferred to a relatively cold and graphite-free analysis chamber and the actual determination of the carbon monoxide content of this gas, which is proportional to the oxygen content of the original sample, takes place within this chamber, no difliculties involving get-tering or contamination are observed. Advantageously, the pressure Within the analysis chamber is elevated to a level conducive to spectroanalysis of the gas (e.g. atmospheric pressure or slightly in excess thereof) and facilitating the reproduction of the analysis results by the admission into the analysis chamber of an inert gas, preferably argon.

According to yet another feature of this invention, the analysis chamber comprises a large-capacity storage receptacle for the evacuated gas and a detection receptacle remote therefrom, circulating means being provided for rapid displacement of the gas stream between these receptacles in a closed path so as to mix the carrier gas with the extracted gas rapidly and completely. The I-R analysis apparatus may include the usual recorder or the like by means of which the steady-state mixture concentration of carbon monoxide within the analysis chamber can be readily observed. Means can also be provided for indicating that the concentration of carbon monoxide in the gas circulated past the spectroanalysis device is constant before the final reading is taken. So that the initial pressure within the analysis chamber after induction of the extracted gas is below the desired measuring pressure, the volume of the analysis chamber may be made somewhat larger (e g. three times as large) than the volume of the extraction chamber, it being noted that the eifective volume of the analysis chamber can include the circulating conduits and storage and detection receptacles.

The above and other objects, features and advantages of the present invention will because more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a diagramma-tical view of the apparatus for carrying out the rapid analysis of steel samples according to the present invention; and

FIG. 2 is a circuit diagram of a control system for automatically or semiautomatically operating the apparatus of FIG. 1.

In FIG. 1 I show an extraction chamber 1 which can be generally similar to those illustrated in my copending application Ser. No. 193,229 and which is amply described therein. For the present purposes, it will be sufficient to note that this chamber 1 has a stationary counterelectrode 5, which can be fed toward the sample electrode 3 by the means described in the copending application upon erosion of the electrodes. The sample electrodes 3 are hollow or can be shifted and distributed around the periphery of a turntable 2, controlled externally of chamber 1, in angularly spaced relationship for successive juxtaposition with the counterelectrode 5. Electrodes 3 receive samples 4 of the metal to be analyzed, these samples having a mass on the order of 1 gram although larger samples can be used, as described. A suction conduit 4% connects chamber 1 with a manometer 11, which indicates the pressure within the chamber 1, and the suction lines 41, 42 connected to a coarse-vacuum pump 7 and a high-vacuum pump 8 which may be of the vapor-lift or diffusion type. Electromagnetic valves 6 and 9 are interposed between pump '7 and 8 and chamber 1. The pump 7 also communicates, in the usual manner, with the high-vacuum pump 8 via an electro-magnetic valve 26, a further manometer 19 indicating the negative pressure produced by pump 7. Suction line 44 also serves as an outlet for the extracted gases and communicates with tube 43 which leads to a storage receptacle 13 of an analysis chamber via a dust filter 17 preventing influx of graphite or metal particles into the analysis chamber. A pair of magnetic valves 15, 16 control the gas flow into the latter chamber, which consists of the storage receptacle 13 and a detection receptacle 1% for the flow analysis of the gas. The analysis chamber further comprises conduits id-43 having valves 19, 2d and 21 and a circulating tube 22 for thoroughly mixing the gas by displacing it between the analysis chamber 18 and the storage chamber 13 in a closed system.

An electromagnetic valve 14 connects the analysis chamber with pump 7 via a conduit 49 while again the valve 25 serves for the efllux of the purging gas. The purging gas can be argon which passes from a tank 50 through valve 24 into the analysis chamber, a further valve 399 being provided for the introduction of a calibrating gas with a known carbon-monoxide content. A pipeline 51 is connected to tank 50, which constitutes a common supply means for both chambers, and to the valve 12 of the extraction chamber 1.

Example A one-gram cylindrical slug of steel cast from the molten metal withdrawn from an open-hearth furnace is analyzed in an apparatus of the foregoing type by interposing it between graphite electrodes and producing an arc discharge between these electrodes with an intensity of 30 amperes. After 5 seconds, the discharge is reduced to a level of about 20 amperes to avoid undue evaporation of metal from the sample, the extraction operation being terminated after about 29 seconds whereupon the extracted carbon monoxide is passed into the analysis chamber. Pure argon is admitted into the latter to raise the pressure therein to several millimeters of mercury above atmospheric pressure, the pressure within this chamber being about 20 mm. of mercury prior to admission of the extracted gas. The extracted gas is circulated in the analysis chamber for 15 seconds whereupon the I-R absorption recorder showed a levelling off of the carbon monoxide content and this value was taken as the carbon monoxide content of the sample. When this method, which was carried out in less than 2 minutes, was compared with chemical methods of analysis taking many times as long, almost identical results were obtained. it should be noted that the intensity of the discharge after melting will be between 20% and 50% of that required to melt the sample and will be determined by the composition of the latter and the type of bonding between the oxygen and the metal.

The invention described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the appended claims.

What is claimed is:

1. A method of determining the oxygen content of a ferrous metal, comprising the steps of:

melting said metal in a closed first chamber by maintaining it for a period of about 10 to 30 seconds in the region of an electric arc formed by at least one carbonaceous electrode to release into said first chamber oxygen contained in said metal, as carbon monoxide; terminating said electric are upon substantially complete release of said oxygen by said metal;

discharging the carbon monoxide thus produced into an evacuated second chamber having a pair of separated compartments immediately upon termination of said electric arc;

admixing the carbon monoxide in said second chamber with an inert gas to increase the total pressure Within said second chamber to a predetermined level by circulating said carbon monoxide and said inert gas between said compartments; and

thereafter analyzing the material within one of said compartments of said second chamber by infrared absorption.

2. Apparatus for determining the oxygen content of a metal, comprising;

an extraction chamber;

a turntable in said chamber formed with a plurality of angularly spaced first arc-discharge electrodes adapted to receive samples of the metal to be analyzed;

a second arcxiischarge electrode in said chamber alignable with said first electrodes;

mechanism for successively juxtaposing said first electrodes with said second electrode, the juxtaposed electrodes forming heating means in said extraction chamber for melting said metal and releasing the oxygen contained therewithin as carbon monoxide;

an analysis chamber communicating with said extraction chamber, said analysis chamber including a storage receptacle,

a detection receptacle remote from said storage receptacle, and

circulating means forming a closed path between said receptacles for continuously circulating gas therebetween;

valve means interposed between said chambers for admitting said carbon monoxide from said extraction chamber into said analysis chamber, said analysis chamber having a volume in excess of that of said extraction chamber;

evacuating means connectible with said analysis chamber for evacuating same prior to the introduction of said carbon monoxide into said analysis chamber;

detection means at said detection receptacle responsive to said carbon monoxide for determining the amount thereof within said analysis chamber;

supply means for feeding an inert gas to said analysis chamber to raise the pressure therein upon admission thereto of said carbon monoxide; and

conduit means communicating between said supply means and said extraction chamber, said evacuating means being connecti'ble with said extraction chamber.

References Cited by the Examiner UNITED STATES PATENTS 2,962,360 11/1960 Bennett et a1 23-253 2,964,389 12/ 1960 Bennett et al 23253 3,065,060 11/1962 Roehrig et a1. 23-253 OTHER REFERENCES Conn, G. K. T., and Avery -D. G.: Infrared Methods, Principles and Applications, pp. 178, 181 and 182 (March 1960).

MORRIS O. WOLK, Primary Examiner.

DELBERT E. GANTZ, Examiner.

H. A. BIRENBAUM, Assistant Examiner. 

1. A METHOD OF DETERMINING THE OXYGEN CONTENT OF A FERROUS METAL, COMPRISING THE STEPS OF: MELTING SAID METAL IN A CLOSED FIRST CHAMBER BY MAINTAINING IT FOR A PERIOD OF ABOUT 10 TO 30 SECONDS IN THE REGION OF AN ELECTRIC ARC FORMED BY AT LEAST ONE CARBOBACEOUS ELECTRODR TO RELEASE INTO SAID FIRST CHAMBER OXYGEN CONTAINED IN SAID METAL, AS CARBON MONOXIDE; TERMINATING SAID ELECTRIC ARC UPON SUBSTANTIALLY COMPLETE RELEASE OF SAID OXYGEN BY SAID METAL; DISCHARGING THE CARBON MONOXIDE THUS PRODUCED INTO AN EVACUATED SECOND CHAMBER HAVING A PAIR OF SEPARATED COMPARTMENTS IMMEDIATELY UPON TERMINATION OF SAID ELECTRIC ARC; ADMIXING THE CARBON MONOXIDE IN SAID SECOND CHAMBER WITH AN INERT GAS TO INCREASE THE TOTAL PRESSURE WITHIN SAID SECOND CHAMBER TO A PREDETERMINED LEVEL BY CIRCULATING SAID CARBON MONAXIDE AND SAID INERT GAS BETWEEN SAID COMPARTMENTS; AND THEREAFTER ANALYZING THE MATERIAL WITHIN ONE OF SAID COMPARTMENTS OF SAID SECOND CHAMBER BY INFRAED ABSORPTION. 